The separation of trace quantities of Cs cations from industrial waste solutions containing other alkali metal cations and/or other chemicals is a difficult, but commercially important separation. Industries where such separations would be advantageous include the semiconductor, nuclear waste cleanup, metals refining, electric power, and other industrial enterprises. The separations are difficult because the Cs to be removed is often present only in concentrations ranging from parts-per-trillion (ppt) to low parts-per-million (ppm) levels and must be separated from other alkali metals that may be present in concentrations up to several molar. Hence, a kinetically rapid, highly selective, and strong thermodynamically interactive material is required for the separations.
Cs and Sr are two of the most important radioactive contaminants in nuclear waste. This is because .sup.137 Cs and .sup.90 Sr contribute about 98% of the thermal energy and 97% of the penetrating radiation during the first thirty years after nuclear waste is formed. It is highly desirable to selectively remove these elements to greatly enhance the safety of and reduce the volume of nuclear waste going to a long term geologic disposal repository for nuclear waste. Furthermore, in dilute radioactive contamination, such as in ground water, Cs and Sr are virtually the only radioactive waste problems requiring treatment. The need for both types of Cs and Sr treatment is found in many sites in the U.S.A. as well as in other countries throughout the world.
In the past, methods for the removal of cesium from nuclear waste streams have been inefficient. A few organic and inorganic ion exchange polymers have been prepared by a variety of methods for Cs separation. One such class of materials is a phenol-formaldehyde type polymer wherein a hydroxybenzene, such as phenol (i.e. CS-100, formerly made and sold by Rohm & Haas) or resorcinol, is reacted with formaldehyde, via hydroxymethylation, and further condensed to form a methylene linkage between benzene rings in the presence of a base or acid to produce a solid, glassy polymer having ion-exchange properties. See, e.g. U.S. Pat. No. 4,423,159. These materials, while functioning somewhat in the complexing of cesium ions, are of limited selectivity. This is particularly true when large concentrations of potassium and sodium are present. The inorganic ion exchange materials, such as crystalline silicotitanates (Sandia National Laboratory) and the ferricyanide-based materials, either lack the selectivity needed, are not elutable, or are not present in a sufficiently stable or practically useful format for effective use.
Calixarenes and related polyhydroxyaromatic molecules are known to have extremely high selectivity with respect to the cesium ion, R. M. Izatt et al., 105 J. Am. Chem. Soc. 1782 (1983); Calixarenes, A Versatile Class of Macrocyclic Compounds (J. Vicens & V. Bohmer eds., 1991); C. D. Gutsche, Calixarenes (1989). To use these molecules to perform separations, however, the molecules must be incorporated into systems where the Cs is selectively involved in a phase change. Previous attempts to involve the polyhydroxyaromatic molecules in Cs separation systems have involved solvent extraction and liquid membrane systems, R. M. Izatt et al., 105 J. Am. Chem. Soc. 1782 (1983); Calixarenes, A Versatile Class of Macrocyclic Compounds (J. Vicens & V. Bohmer eds., 1991); C. D. Gutsche, Calixarenes (1989). These systems have the disadvantages of the use of an organic solvent in the system, relatively slow kinetics, loss of efficiency as the Cs feed concentration decreases, loss of the costly molecule to the aqueous phases, formation of emulsions during the separation, and other difficulties. Moreover, these materials are quite hydrophobic and do not always retain the necessary properties for use in separating cesium from an aqueous system.
It would be desirable to formulate hydroxyaromatic ligands into a stable hydrophilic polymeric solid resin wherein the selective properties of the hydroxyaromatic ligands for cesium cations are maintained in an actual separation system and wherein the ligands can be reused efficiently with rapid kinetics hundreds or thousands of times to make separations. The reuse of such ligands makes their use economical and of significant industrial worth. These objectives are accomplished by means of the condensation of formaldehyde with a poly(hydroxyarylene) ligand and, optionally, other alkoxy- or hydroxy-aromatic compounds or methylated hydroxyaromatic compounds to form a polymeric resin and the use of such poly(hydroxyarylene)-containing polymeric resins in actual separation processes.